Bump the minimum recommended Payara version to 5.2021.6

[landreev]

We now have a warning about the logging configuration bug in 5.2021.5 (in doc/sphinx-guides/source/installation/installation-main.rst). But we still have this version explicitly mentioned as the minimum recommended in doc/sphinx-guides/source/installation/prerequisites.rst, and mentioned explicitly in doc/sphinx-guides/source/developers/dev-environment.rst.

It appears to be safe and uncontroversial to bump it up to 5.2021.6. If I understand it correctly, @qqmyers has been running it successfully on a prod. instance (correct me if I'm wrong though!)

[qqmyers]

QDR is already at 5.2021.8. I've run 5.2021.6, 5.2021.7, 5.2021.10 and 5.2022.1 in dev instances as well - haven't seen any differences other than the issue of having to clear the H2 database dir for 5.2021.8+ if upgrading. I think others have upgraded to 5.2021.8+ since we've had questions about that H2 problem.